Neoplasia is a process that occurs in cancer, by which the normal controlling mechanisms that regulate cell growth and differentiation are impaired, resulting in progressive growth. This impairment of control mechanisms allows a tumor to enlarge and occupy spaces in vital areas of the body. If the tumor invades surrounding tissue and is transported to distant sites it will likely result in death of the individual.
The desired goal of cancer therapy is to kill cancer cells preferentially, without having a deleterious effect on normal cells. Several methods have been used in an attempt to reach this goal, including surgery, radiation therapy, and chemotherapy.
Local treatments, such as radiation therapy and surgery, offer a way of reducing the tumor mass in regions of the body that are accessible through surgical techniques or high doses of radiation therapy. However, these treatments are not applicable to the destruction of widely disseminated or circulating tumor cells eventually found in most cancer patients. In order to combat the spread of tumor cells, systemic therapies are used.
The primary weapon against cancer is chemotherapy. However, chemotherapeutic agents are limited in their effectiveness for treating many cancer types, including many common solid tumors. This failure is in part due to the intrinsic or acquired drug resistance of many tumor cells. Another drawback to the use of chemotherapeutic agents is their severe side effects. These include bone marrow suppression, nausea, vomiting, hair loss, and ulcerations in the mouth.
Reductive prodrugs are compounds that are nontoxic in their native form, but produce a highly toxic species when reduced. These drugs kill by generating DNA adducts and can target both growing and non-growing tumor cells, which is advantageous since in human tumors, generally only a small fraction of cells are actively replicating at a given time. Reductive prodrug cancer chemotherapy with compounds such as MMC and CB 1954 owes its rationale to the fact that he concentration of the enzymes that reduce them, such as mammalian DT-diaphorase (NQO1) increases in tumor cells. This makes the tumor cells more potent reducers of these drugs, and therefore more susceptible to their killing effect. However, these enzymes are present also in normal mammalian cells, and while their activity is lower in such cells than in tumor cells, it is high enough to produce severe side effects.
One approach to preferentially killing pathological cells, most widely used for treating cancer, is to introduce a gene into the target cells that encodes an enzyme capable of converting a prodrug of relatively low toxicity into a potent cytotoxic drug. Systemic administration of the prodrug is then tolerated since it is only converted into the toxic derivative locally, for example in a tumor, by cells expressing the prodrug-converting enzyme. This approach is known as gene-directed enzyme prodrug therapy (GDEPT), or when the gene is delivered by means of a recombinant viral vector, virus-directed prodrug therapy (VDEPT) (McNeish et al, 1997). A class of enzymes that has been well studied in GDEPT is bacterial nitroreductases (NTRs), such as NfsA and NfsB from Escherichia coli. These enzymes can reduce several nitro substituted organic compounds.
An example of an enzyme/prodrug system is nitroreductase and the aziridinyl prodrug CB1954 (5-aziridinyl-2,4-dinitrobenzamide) (Knox et al 1988). CB1954 is a poor substrate for the human nitroreductase, and so GDEPT was conceived as a way of introducing a suitable nitroreductase, preferably with greater activity against CB1954, in order to sensitize targeted cells. The E. coli nitroreductase has been widely used for this purpose.
Targeted biological therapies hold tremendous potential for the treatment of cancers, yet their effective use has been limited by constraints on delivery and effective tumor targeting. There exists a need for a local therapy that provides for effective killing of tumor cells. The present invention addresses this need.
Relevant Literature
Pawelek et al. (1997) Cancer Research, Vol 57, Issue 20 4537-4544. European Organisation for Research and Treatment of Cancer “Molecular Targets and Cancer Therapeutics” Conference, Philadelphia, Pa. Nov. 14-18 2005. Poster A33: Improvement of a Novel Enzyme, Using Directed Evolution, for Reductive Cancer Chemotherapy, Barak et al. Ackerley et al. Environ Microbiol. August 2004;6(8):851-60, “Mechanism of chromate reduction by the Escherichia coli protein, NfsA, and the role of different chromate reductases in minimizing oxidative stress during chromate reduction”. Ackerley et al. (2004) Appl Environ Microbiol. February;70(2):873-82, “Chromate-reducing properties of soluble flavoproteins from Pseudomonas